1. Field of the Invention
The present invention relates to a differential amplifier and a semiconductor integrated circuit for LCD drive which amplifies differential input signals to output an amplified output.
2. Related Background Art
The conventional differential amplifying circuit is generally realized by using a differential pair of transistors. There is a problem in which operational range of a common mode voltage of the differential input signals is limited due to influence of gate-source voltages Vgs of MOS transistors composing the differential pair.
As a method of solving such a problem, U.S. Pat. No. 4,554,515 and Japanese Patent document No. H4-76246 discloses a method of adding current of two differential pairs of conduction types different from each other in order to enlarge operational range of the common mode voltage of the differential input signals.
However, there are the following problems 1)–3) in this method.
1) In accordance with size of the common mode voltage of the differential input signal, the respective input conversion offsets corresponding to two differential pairs are added and output deviation becomes large.
2) Because directions of the respective output current corresponding to two differential pairs are in reverse to each other, a current addition circuit must be provided. Because of this, the number of components increases, thereby enlarging circuit volume.
3) Sum of the differential output current fluctuates depending on an operational point of the common mode voltage of the differential input signals.
On the other hand, Japanese Patent Laid Open No. H8-256026 discloses a method of adding currents corresponding to the outputs of the differential pair with the same conduction type and different threshold voltages in order to enlarge operation range of the common mode voltages of the differential input signals.
FIG. 17 is a circuit diagram of the differential amplifying circuit disclosed in the H8-256026. The differential amplifying circuit of FIG. 17 has a first differential pair composed of NMOS transistors M1 and M2, a second differential pair composed of NMOS transistors M3 and M4, an NMOS transistor M31 composing a current source for supplying current to the first differential pair, an NMOS transistor M32 composing a current source for supplying current to the second differential pair, PMOS transistors M15 and M16 composing an active load connected to the output terminals of the first and second differential pairs, and a PMOS transistor M34 and an NMOS transistor M33 composing an output circuit.
However, because the differential amplifying circuit of FIG. 17 supplies current from separate current sources to the respective differential pairs, there is the same problem as that that of the above 3), i.e. sum of the differential output currents fluctuates depending on the operational point of the common mode voltages of the differential input signals. Because of this, a stable output is not obtained, and the operational range of the common mode voltages of the differential input signals is not so much enlarged.